Changes in Southern Piedmont Grassland Community Structure and Nutritive Quality Under Future Climate Predictions: Tropospheric Ozone and Altered Rainfall

Abstract

Tropospheric (ground-level) ozone (O3) is the most significant phytotoxic air pollutant in the United States. It is readily transported to rural agricultural and forested areas from urban centers. Estimating the effects of elevated ozone concentrations in combination with altered rainfall amounts is important for predicting effects of future climatic changes. The Southern Piedmont region spans 17 million hectares from central Virginia to eastern Alabama. There are numerous reports demonstrating ozone-induced injury to foliage, reduction in crop growth, and yield losses in economically important plants in this region. Only recently have there been studies to determine the effects of elevated ozone concentrations on wild plants. In addition, experiments have been conducted to explore ozone-induced alterations in nutritive quality regarding mammalian herbivores. To determine the effects of ozone on grassland forage species as well as effects on herbivore nutrition, two independent experiments were designed. In the first experiment several forage species common to the region were examined for effects of ozone exposure in combination with varying rainfall amounts: tall fescue (Lolium arundinacea), dallisgrass (Paspalum dilatatum), common bermudagrass (Cynodon dactylon), and ladino clover (Trifolium repens). These forages were exposed to two levels of ozone in a randomized split-plot experiment with three levels of precipitation. Plants were grown in open-top chambers from June – Sept 2009 under non-filtered (NF) and twice-ambient (2X) ozone concentrations. Precipitation regimes were in three blocks and represented average (30-yr average for Auburn, AL); high (+ 20% of average rainfall); and low (- 20% of average). These differing ozone concentrations and rainfall amounts were representative of future predicted climatic changes. Interestingly, primary-growth (representing cumulative ozone-exposure effects) grasses differed significantly in biomass over the growing season across precipitation and ozone treatments, with 2X treatments having greater biomass. The primary-growth grasses experienced little differences in nutritive quality over the growing season between ozone treatments. Regrowth (representing one month of exposure) grasses actually responded positively to ozone effects, with lower neutral-detergent fiber (NDF) concentrations and greater relative feed value (RFV) than NF-exposed grasses throughout the growing season. Mean nutritive quality at final harvest for regrowth grasses reflected similar effects, with RFV greater in 2X treatments. Primary-growth clover exhibited decreased nutritive quality in 2X treatments with increases in NDF, ADF, and significant increases in lignin concentrations. Regrowth clover exposed to 2X ozone treatments demonstrated decreased nutritive quality; i.e. increased concentrations of cell-wall constituents (ADF and NDF) and lower RFV in the final harvest as well as over the growing season. Total plant canopy cover also decreased more rapidly at the end of the growing season in 2X-exposed communities compared with NF communities. Decreases in nutritive quality of certain forages have implications for herbivores that rely on these species for energy and nutrient consumption. Also, if grassland communities contain forage species that are “sensitive” to greater concentrations of ozone, grassland community structure and diversity could be altered. Regarding experiment two, we hypothesized that exposure to elevated ozone concentrations would alter the chemical composition of forages affecting nutrient utilization by lagomorphs. In a balance trial (feeding stalls in the lab), New Zealand White rabbits (Oryctolagus cuniculus) were fed forages grown under differing ozone levels, NF and 2X, and harvested from OTCs. The rabbit is an ideal and ecologically relevant model herbivore for these experiments because rabbits utilize similar selective foraging and digestive strategies as most wild and domestic mammalian herbivores. The feeding trial was initiated in January 2010, with rabbits receiving forages grown in different ozone-treatment chambers (NF and 2X) for a total of 10 days. Neutral-detergent fiber (NDF) and acid-detergent fiber (ADF) digestibility by rabbits were significantly lower for 2X than NF diets. The decreased digestibility was not attributed to lignin concentration, but was significantly correlated with increased saponifiable phenolic concentrations. Digestible dry matter intake (DDMI) of rabbits receiving 2X diets was also significantly decreased compared with rabbits receiving NF diets. Elevated ozone was determined to increase bound-phenolic concentrations in forages that were associated strongly to decreased digestibility of forages. Based on the above results from the two independent experiments, elevated ozone concentrations appear to have a negative impact on forage quality, resulting in decreased nutrient utilization by herbivores in Southern Piedmont grassland communities.